Nontypable H. influenzae are a frequent cause of localized respiratory tract infections, including otitis media, sinusitis, bronchitis, and pneumonia. In addition, these organisms are an important cause of serious systemic infections, especially in immunocompromised hosts. The initial step in the pathogenesis of disease due to nontypable Haemophilus involves colonization of the upper respiratory tract. The investigators have demonstrated that two proteins designed HMW1 and HMW2, the prototypic members of a family of antigenically- related high-molecular-weight proteins present in most isolates of nontypable H. influenzae, mediate attachment to human epithelial cells. Based on their in vitro results, they speculate that members of this family are important colonization factors. The objective of this proposal is to characterize the molecular mechanism of the interaction between HMW1 and HMW2 and host epithelium. Initially, the process by which these proteins are presented in an active binding conformation on the surface of the organism will be defined. Subsequently, the domains of HMW1 and HMW2 involved in recognition of host cells will be identified. Finally, to confirm a role of these proteins in in vivo colonization, mutants deficient in expression of HMW1 or HMW2 or both will be examined in an animal colonization model. From a practical perspective, the results of these studies may suggest novel strategies for the prevention of nontypable H. influenzae disease. More generally, they may provide fundamental insights into the nature of the host-microbial relationship and the mechanism of protein section.